Information
-
Patent Grant
-
6548945
-
Patent Number
6,548,945
-
Date Filed
Friday, October 20, 200023 years ago
-
Date Issued
Tuesday, April 15, 200321 years ago
-
Inventors
-
Original Assignees
-
Examiners
- Kim; Robert H.
- Gemmell; Elizabeth
Agents
-
CPC
-
US Classifications
Field of Search
US
- 313 141
- 313 143
- 123 169 A
- 445 7
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International Classifications
-
Abstract
A spark plug has a tubular fitting which is assembled with a center electrode, a ground electrode and an insulator. The fitting has a reach length of at least 12 mm for use in high output-type engines. The fitting is formed with a thread part and a taper part so that the fitting is thread engaged with an engine head and seals a combustion chamber from an outside by a contact between the taper part and a seat surface of the engine head. The fitting is made by cold-forging a low carbon steel to provide the taper surface of a surface roughness of less than 10 μm and a column part. The column part is machine-cut to provide the thread part thereon. The deflection between the axes of the thread part and the taper part is limited to be less than 0.15 mm.
Description
CROSS REFERENCE TO RELATED APPLICATION
This application is based on and incorporates herein by reference Japanese Patent Application No. 11-300208 filed Oct. 21, 1999.
BACKGROUND OF THE INVENTION
The present invention relates to a spark plug that is inserted into a combustion chamber of an engine, and suitable for use in an engine that has a thick engine head for a higher engine output.
In conventional spark plugs, as shown in
FIG. 3
, a cylindrical insulator
301
surrounding a center electrode
305
is held inside a metallic fitting
303
in such a manner that one end
302
of the insulator
301
protrudes from one end
304
of the fitting
303
. A ground electrode
307
is fixed to the fitting
303
so that the ground electrode
307
faces the top end of the center electrode
305
protruding from the insulator
301
through a discharge gap
308
. This spark plug is threaded into a thread hole
310
formed in an engine head
309
that defines a combustion chamber
320
therein.
The fitting
303
is formed, on its outer peripheral surface, with a thread part
311
and a taper part
312
from the side of the end
304
. The thread part
311
is engaged with the thread hole
310
by turning the plug. The taper part
312
has a diameter gradually decreasing toward the thread part
311
. The taper part
312
contacts a taper surface
313
formed on the thread hole
310
to restrict leakage of gas from the combustion chamber
320
.
The fitting
303
is produced by a cold-forging and then machine-cut to form the taper part
312
and the thread part
311
in shape. The machine-cutting tends to produce traces of a cutting tool (tool mark) on the taper part surface, resulting in a high surface roughness. Further, the machine-cutting tends to produce deflection of longitudinal axes between the thread part
311
and the taper part
312
. As a result, sealing characteristics of the taper part
312
is lessened.
In high output-type engines, the engine head is made thicker to ensure more coolant flow for higher cooling efficiency. The fitting of the spark plug for such engines generally has a reach length RL of more than 12 mm. As known well in the art, the reach length is defined as a length from the end
304
of the fitting
303
to the point where the diameter of the taper part
312
is 14.8 mm. In the case of a spark plug having a longer reach length for high output type engines, it must have a sufficient sealing ability because the pressure in the combustion chamber
320
increases.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a spark plug that can provide a sufficient sealing ability for use in high output-type engines.
According to the present invention, a spark plug comprises a center electrode, a ground electrode and a tubular fitting for engagement with an engine head. The fitting has a thread part and a taper part from one end thereof toward another end thereof. The fitting is formed by cold-forging a low carbon steel so that the taper part has its cold-forged surface roughness of less than about 10 μm. The fitting is machine-cut to provide the thread part which deflects less than about 0.15 mm from the taper part with respect to longitudinal axes. This fitting is suitably used for spark plugs for high output engines in which a reach length of the fitting is at least 12 mm.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings:
FIG. 1
is a front view showing, partly in section, a spark plug according to an embodiment of the present invention;
FIG. 2
is a graph showing a relationship among a surface roughness, a deflection amount and an air leakage amount; and
FIG. 3
is a front view showing, partly in section, a part of a conventional spark plug.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring first to
FIG. 1
, a spark plug
100
is threaded into a thread hole
201
formed in an engine head
200
that defines a part of combustion chamber F
1
. The spark plug
100
has a generally tubular fitting
10
made of a conductive steel material (for instance, low carbon steel). On the outer peripheral surface of the fitting
10
, a thread part
13
, a taper part
14
and a hexagonal nut part
15
are formed from one end
11
at the combustion chamber side to the other end
12
. The plug
100
is fixedly inserted by engaging the thread part
13
with the thread part
201
while turning the hexagonal nut part
15
by a wrench or like tools.
The taper part
14
has a diameter that decreases gradually toward the thread part
13
in the axial direction. The taper part
14
tightly contacts a tapered seat surface
202
formed on the thread hole
201
, thus restricting leakage of gas from the combustion chamber F
1
. The fitting
10
has a reach length RL of more than 12 mm. The reach length RL is defined as an axial length from the end
11
to a point where the diameter of the taper part
14
is 14.8 mm.
The fitting
10
tightly holds therein a cylindrical insulator
20
made of alumina ceramics such as AL
2
O
3
. One end
21
and the other end
22
of the insulator
20
are exposed from the one end
11
and the other end
12
of the fitting
10
, respectively. Packings
23
and
24
are interposed between the insulator
20
and the fitting
10
to seal a space between the insulator
20
and the fitting
10
. Specifically, the packing
23
is located near the end
11
of the fitting
10
, and the packing
24
is located right at the other end
12
of the fitting
10
.
The insulator
20
fixedly holds therein a center electrode
30
and a stem
40
that are connected to each other. One end
31
of the center electrode
30
protrudes from the end
21
of the insulator
20
, and one end
41
of the stem
40
protrudes from the other end
22
of the insulator
20
. Thus, the center electrode
30
is insulated from the fitting
10
by the insulator
20
and protrudes into the combustion chamber F
1
.
A ground electrode
50
is fixed to the end
11
of the fitting
10
by welding or the like. The ground electrode
50
is formed in the L-shape and faces the end surface of the center electrode
30
through a discharge gap
60
. The spark plug
100
thus generates a spark discharge to ignite air-fuel mixture in the combustion chamber F
1
when a high discharge voltage is applied between the center electrode
31
and the ground electrode
50
.
In this embodiment, the fitting
10
is produced by a cold-forging into a shape that has the taper part
14
and a columnar part for the thread part
13
. Machine-cutting is applied only to the columnar part to form the thread part
13
. It is preferred that the fitting
10
is made of a carbon steel material which includes carbon in as low percentage as possible. No cutting trace is produced on the taper part surface, because the taper part
14
is not machine-cut. The deflection (lateral offset) of longitudinal axes of the taper part
14
and the thread part
13
is minimized, because the taper part
14
and the columnar part for the thread part
13
are produced by using the same die in the cold-forging process.
The surface roughness of the taper part
14
can be improved by lowering the surface roughness of the die used for the cold-forging so that the sealing ability of the taper part
14
and the seat surface
202
of the head
200
is increased. The taper part
14
is enabled to contact the seat surface
202
uniformly over an entire circumference of the taper part
14
, because the axes of the thread part
13
and the taper part
14
are aligned in line with a least deflection (offset).
The sealing ability of the taper part
14
formed by the cold-forging is set to have the following characteristics for spark plugs that have the reach length RL of 12 mm or more. That is, the amount of gas (air) leaking from the combustion chamber F
1
out to outside through the taper part
14
should be less than 1 cm
3
per minute under a condition that the spark plug is mounted as shown in
FIG. 1
, the pressure of gas in the combustion chamber F
1
is 1.96 Mpa (20 kg/cm
2
) and the temperature at the taper part
14
is 200° C. This sealing ability cannot be attained by such conventional spark plugs as shown in FIG.
3
.
The result of study on the sealing ability is shown in
FIG. 2
in relation to the surface roughness of the taper part
14
and the deflection (D) of the axes of the thread part
13
and the taper part
14
. The surface roughness is measured according to JIS B0651-1996 by using a needle tip end of 2 μm, that is, by using a surface roughness meter and defining the roughness according to a 10-point average method.
As understood from
FIG. 2
, the leakage of gas decreases as the surface roughness decreases. This is because lower roughness produces less friction between the taper part
14
and the seal surface
202
and enables the fitting
10
to be screwed into the thread hole
201
deeper thereby to increase the tightening force in the axial direction, when the fitting
10
is screwed into the thread hole
201
. It is clear from
FIG. 2
that the surface roughness should be less than 10 μm to restrict the leakage to be less than 1 cm
3
/min.
As also understood from
FIG. 2
, the leakage of gas decreases as the deflection D decreases. This is because less deflection produces less local friction between the taper part
14
and the seat surface
202
and enables tightening of the fitting
10
deeper into the thread hole
201
. It is clear from
FIG. 2
that the leakage can be maintained to be less than 1 cm
3
/min, as long as the deflection D is less than 0.15 mm if the roughness is less than 10 pm. The deflection, or lateral spacing between longitudinal axes, D is more preferably less than 0.1 mm.
The present invention should not be limited to the above embodiment, but may be modified in many other ways without departing from the spirit of the invention.
Claims
- 1. A spark plug for engines comprising:a tubular fitting having a thread part and a taper part in that order on an outer surface of the tubular fitting, from one end thereof toward another end thereof and having a reach length of at least 12 mm, the thread part being for engagement with the engine, and the taper part being for sealing a combustion chamber from an outside by contacting the engine; a center electrode held in and insulated from the tubular fitting with one end thereof protruding from the one end of the tubular fitting; and a ground electrode fixed to the tubular fitting and facing the one end of the center electrode through a discharge gap therebetween, wherein the taper part is formed into a tapered shape thereof by a cold-forging.
- 2. The spark plug as in claim 1, wherein the taper part has a surface roughness of less than about 10 μm.
- 3. The spark plug as in claim 2, wherein a lateral spacing between longitudinal axes of the taper part and the thread part is less than about 0.15 mm.
- 4. The spark plug as in claim 1, wherein a lateral spacing between longitudinal axes of the taper part and the thread part is less than about 0.15 mm.
- 5. The spark plug as in claim 1, wherein the taper part is not machine-cut after the cold-forging and the thread part is machine-cut after the cold forging.
- 6. The spark plug as in claim 1, wherein said taper part is disposed adjacent said thread part, and said taper part has a first outer diameter at a first end thereof adjacent said thread part generally corresponding to an outer diameter of said thread part, and has a tapered surface to a second end thereof which has a second outer diameter greater than said first outer diameter and greater than a maximum diameter of said thread part.
- 7. The spark plug as in claim 1, wherein the taper part has a tapered outer surface of gradually increasing diameter from a first diameter adjacent said thread part to a second diameter, larger than the first diameter, at an end thereof remote from said thread part, said second diameter being greater than a maximum diameter of said thread part.
- 8. A spark plug for engines comprising:a tubular fitting having a thread part and a taper part in that order on an outer surface of the tubular fitting, from one end thereof toward another end thereof and having a reach length of at least 12 mm; a center electrode held in and insulated from the tubular fitting with one end thereof protruding from the one end of the tubular fitting; and a ground electrode fixed to the tubular fitting and facing the one end of the center electrode through a discharge gap therebetween, wherein the taper part of the tubular fitting has a cold-forged surface roughness of less than about 10 μm.
- 9. The spark plug as in claim 8, wherein a lateral spacing between axes of the taper part and the thread part is less than about 0.15 mm.
- 10. The spark plug as in claim 9, wherein the lateral spacing between the axes of the taper part and the thread part is less than 0.1 mm.
- 11. The spark plug as in claim 8, wherein the taper part is not machine-cut after the cold-forging and the thread part is machine-cut after the cold forging.
- 12. The spark plug as in claim 8, wherein said taper part is disposed adjacent said thread part, and said taper part has a first outer diameter at a first end thereof adjacent said thread part generally corresponding to an outer diameter of said thread part, and has a tapered surface to a second end thereof which has a second outer diameter greater than said first outer diameter and greater than a maximum diameter of said thread part.
- 13. The spark plug as in claim 8, wherein said thread part is machine cut to define a threaded outer circumferential surface for engaging a thread hole of the engine so that when the thread part is engaged with the thread hole, a contact between the taper part and a seat surface of the thread hole seals the thread hole.
- 14. The spark plug as in claim 8, wherein the taper part has a tapered outer surface of gradually increasing diameter from a first diameter adjacent said thread part to a second diameter, larger than the first diameter, at an end thereof remote from said thread part, said second diameter being greater than a maximum diameter of said thread part.
- 15. A method of manufacturing a spark plug for an engine having a thread hole comprising:cold-forging a low carbon steel into a shape of a tubular fitting having a taper part, which contacts the thread hole of the engine to seal a combustion chamber from an outside, and a column part, the taper part having a cold-forged surface roughness of less than about 10 μm; and assembling a center electrode and a ground electrode with the tubular fitting.
- 16. The method as in claim 15, further comprising:machine-cutting only the column part to form a thread part thereon which is engageable with the thread hole of the engine.
- 17. The method as in claim 16, wherein a lateral spacing between longitudinal axes of the taper part and the thread part is less than about 0.15 mm.
- 18. The method as in claim 17, wherein the fitting is sized to have a reach length of about more than 12 mm from one end thereof to a point on the surface of the taper part where the diameter of the taper part is 14.8 mm.
- 19. The method as in claim 16, wherein said taper part is disposed adjacent said thread part, and said taper part has a first outer diameter at a first end thereof adjacent said thread part generally corresponding to an outer diameter of said thread part, and has a tapered surface to a second end thereof which has a second outer diameter greater than said first outer diameter and greater than a maximum diameter of said thread part.
- 20. The method as in claim 16, wherein the taper part has a tapered outer surface of gradually increasing diameter from a first diameter adjacent said thread part to a second diameter, larger than the first diameter, at an end thereof remote from said thread part, said second diameter being greater than a maximum diameter of said thread part.
Priority Claims (1)
Number |
Date |
Country |
Kind |
11-300208 |
Oct 1999 |
JP |
|
US Referenced Citations (2)
Number |
Name |
Date |
Kind |
5088311 |
Inoue |
Feb 1992 |
A |
5581145 |
Kato et al. |
Dec 1996 |
A |
Foreign Referenced Citations (3)
Number |
Date |
Country |
59-130391 |
Sep 1984 |
JP |
60-133592 |
Sep 1985 |
JP |
2000-48930 |
Feb 2000 |
JP |